1 Introduction
With the development of science and technology, higher and higher requirements are put forward for cutting processing. First, it is necessary to meet higher and higher processing efficiency, processing precision and surface quality. Secondly, it requires economical and ecological (ie green production requirements). . In order to meet these requirements, the researchers have done a lot of research work and developed a variety of advanced cutting technology, such as high-speed cutting, dry cutting, hard cutting and so on.
The system was first developed in the 1960s on integrated circuit (IC) manufacturing and materials research. Therefore, the manufacturing technology used in the development must follow the manufacturing requirements of integrated circuits, and the materials used must also comply with the integration. The manufacturing standards of the circuit include the use of silicon-based materials such as polysilicon, single crystal silicon, silicon oxide, and silicon dioxide, or the use of metals such as aluminum and copper. However, with the diversification of MEMS and micromachines, materials that traditionally meet the requirements for integrated circuit manufacturing have their limitations, and the need for microcomponents with different mechanical properties and electronic properties is becoming more and more urgent. MEMS technology has become one of the fastest growing industries in the world, and there is a large demand for industries that need to manufacture extremely small, high-precision parts, such as biology, medical equipment, optics, and microelectronics, including mobile communications and computer components. . However, not every micro-component used in MEMS or micro-mechanical applications can be produced using integrated circuit technology, so new materials and new micro-fabrication technologies and micro-cutting technologies have been developed.
2 scale division
For the division of scales, researchers from different research institutions and different research fields have different opinions. Material science experts believe that the scale between 10 and -12 powers m to 10 and the -9th power m belongs to the field of quantum mechanics research; the scale between 10 and -9 powers m to 10 and -6 power squares belongs to The scope of the research on nanoscopic mechanics; the scale between 10 and the -6th power m~10 to the power of -3 is the category of mesoscopic mechanics; the scale between 10 and -3 powers m to 10 and the power of 0 It belongs to the field of micro-mechanical research; the scale of 0 to the power of m is a category of macro-mechanical research. The machining discipline often uses 10 to 6 power m (1 μm) as the machining error scale. The error scale of traditional machining is mostly measured by wire (1 wire = 10 μm), and the error scale of precision machining can reach the micrometer level. It can be seen that the material science uses the feature length of the research object as the basis for the scale division. The machining field uses the processing precision of the research object as the basis for the division of the scale, so that the machining is divided into ordinary machining, precision machining and ultra-precision machining. There is no reference to the size of the workpiece processing feature scale.
Precision machining can be divided into macro-scale machining, mesoscale machining and micro-scale machining according to the dimensions of workpiece machining features. The general machining process mostly refers to macro-scale machining. The technical performance requirements of the parts are reflected on the macro-scale or surface structure. The size of the machining features is relatively large, and the processing category is wide. Micro-scale machining refers to micro-nano processing, mainly used. Precision and ultra-precision machining technology, micro-machining technology and nano-machining technology, emphasizes "very thin cutting" and microstructure, processing features are relatively small in size, in micro, sub-micron and nano-scale, research focus It is the microstructure of matter; between the two is called mesoscale processing or medium-scale processing.
At present, some electromechanical products are not as small as microelectromechanical systems (micromachines) in nanotechnology, and are not as large as ordinary electromechanical products. They can be called "micromachines" for easy differentiation. The processing characteristics of micromachines span a number of different scales, including a microscale between 10 and 3 to the power of 0 to 10, and 10 to -6 to m to 10 The mesoscopic scale between squares m also includes a nanoscale between 10 and -9 powers m to 10 and -6 power squares. It should be pointed out here that the processing precision that most microfabrication technologies can achieve is still in the submicron to micrometer range, which is called the nanometer scale (10 to -10 powers m to 10 to -7th power m). There is still a big gap.
Micromachines have large markets in defense, aerospace, aerospace and civilian applications, such as tiny satellites, aircraft, machine tools, steam turbine generators, vehicles, and firearms. From the perspective of product development, miniaturization is one of its directions. Cameras, cameras, projectors, mobile phones, etc. are getting smaller and smaller, but the functions are constantly improving and perfecting. Therefore, the research of micromachining theory and technology has broad application prospects.
3 micro-manufacturing technology
Micromanufacturing, which is currently used in MEMS, can be divided into silicon-based materials and micro-machining of non-silicon-based materials, which can be basically divided into four categories:
(1) Etching technology
The technique utilizes dry etching, wet etching or photolithography to perform isotropic or non-isotropic etching on the material to be processed, and generally performs bulk micromachining or surface micro-processing on the material to be processed. Surface micromachining. The advantages of etching technology are high processing precision and large-scale production capacity, which is compatible with IC manufacturing, and the technology is mature. The disadvantages are fixed materials, slow processing speed, high etchant risk, and equipment funds. The investment is large and the processing environment is demanding.
(2) Thin film technology
This technology mainly uses the film growth technology and etching technology to process the required microstructures. It is generally used for 2D surface micromachining and is mainly used in the manufacture of micro devices for VLSI. In addition to the mature technology and excellent IC compatibility, the thin film technology can produce a large number of micro-components without special assembly technology, and its disadvantages are the same as those of the etching technology.
(3) LIGA technology
This technology combines technologies such as Deep X-Ray lithography, Micro electroforming and Micro molding. LIGA micromachining technology has high precision and good surface roughness. In addition to the advantages of good IC circuit compatibility and mass production, LIGA technology can process a wider variety of materials and better high aspect ratio 3D microstructure manufacturing capabilities than IC manufacturing technology. However, the biggest disadvantage of LIGA technology is that the cost of synchrotron X-ray required for manufacturing is extremely expensive, and the cost and time of X-ray mask are also high, so it has been used in submicron-scale microstructures. The use of cheaper LIGA-like technology to replace X-ray etching, such as UV lithography with alternative sources, excimer laser processing, and reactive ion etching (RIE), these alternatives Although the processing precision of the technology is not high in LIGA technology, the light source equipment is small and the price is relatively cheap.
(4) Micromachining technology
In addition to the above-mentioned (1) to (3) micro-manufacturing technology, most of them can be attributed to such processing technology, and micro-machining technology can be divided into three categories: micro-machining technology, non-cutting processing and special processing. This article mainly introduces micro-cutting technology.
4 micro cutting technology
Micro-cutting is a fast and low-cost machining method for small parts. It is not limited by materials. CNC machining centers can be used to realize 2D and 2.5D simple features to micro-machining of complex 3D curved parts. The tiny mold can achieve the purpose of mass production. The following mainly introduces micro-cutting equipment, tools, and cutting mechanisms.
4.1 Micro cutting equipment
The size and quality of the part (machining accuracy, surface roughness, repeatability) are closely related to the performance of the machine tool (such as accuracy, dynamics, etc.). The performance of the machine tool is mainly related to the spindle, the table and the control system. The diameter of the tool used for micro-cutting is very small. In order to improve the machining efficiency, the spindle speed of the micro-cutting machine is very fast. In order to meet the torque requirements, electric spindles and hybrid angular contact bearings are usually used. These bearings have thermal expansion due to frictional heat generation, and the maximum speed generally does not exceed 60,000 r/min. When the speed is higher, the air bearing should be used, but the air bearing provides less torque. At present, the maximum speed of the air bearing spindle can reach 200,000 r/min. In order to achieve a higher cutting speed, the taper of the spindle coincides with the taper of the high speed cutting shank. The table of the micro-cutting precision machine tool is generally driven by a linear motor. Compared with the ordinary drive such as the ball screw, the linear motor drive system has no cumulative error caused by friction and electromagnetic coupling, and there is no loss of precision due to wear and tear. The gap, and can provide a large acceleration, the accuracy of the linear motor drive system can reach ±1μm. Micro-cutting precision machine tools have good stiffness, low vibration, and most come with a variety of sensors and actuators. However, due to its large size, the control requirements for the surrounding environment are relatively strict, making the cost of processing small parts higher.
Due to the small size of the machining features of tiny mechanical products, researchers are trying to develop tiny machine tools to machine small parts. Small machine tools are very small in size and can save a lot of raw materials, so they can be made from materials with better performance. In addition, due to the small mass, the natural frequency of the tiny machine tool is higher than that of the ordinary machine tool, which enables the small machine tool to stably use it over a wide range of spindle speed without flutter. Even if vibration occurs, the amplitude of the minute machine tool is small under the same load. The positioning accuracy of the tiny machine tool can reach the nanometer scale, and the machining precision is submicron.
The development of tiny machine tools has introduced a new concept called “miniature plantsâ€. The miniaturization plant has a very small footprint and can be placed anywhere in any building, even on the battlefield or in a space station, which is almost impossible for ordinary machine tools. Miniaturization plants consume very little energy and greatly save energy use. There are different production units in the mini-chemical plant, such as mini lathes and micro- milling machines .
The development of tiny machine tools currently faces a number of challenges, such as the need to develop sensors and actuators of sufficiently small size to be installed in tiny machine tools. The rigidity of small machine tools is not as good as that of micro-cutting precision machine tools. In addition, in order to prevent external interference, small machine tools need to add vibration isolation devices to meet the processing accuracy requirements. Reducing the processing costs of miniaturization plants and developing multi-functional composite micro-machine tools are the future development trend of micro-cutting equipment.
4.2 Micro cutting tool
In the field of micro-machining technology, how to refine the tool material grain and miniaturize the tool to process the micro-workpiece has always been the focus of research.
The cutting depth and feed rate of the micro-cutting are very small, so the cutting force per unit cutting area is large, and at the same time, a large amount of heat is generated, so that the temperature of the local portion of the cutting edge tip is raised, so the performance of the micro-cutting tool material is improved. The requirements are high, and it is necessary to use tool materials with high wear resistance, heat resistance, high temperature hardness and high temperature strength. With the miniaturization of the minimum diameter of the rotation, the bending strength, rigidity and fracture toughness of the rotary cutter are required to be high. Micro-cutting tool materials are mainly made of hard alloy (tungsten carbide), PCBN (stere boron nitride) and diamond. Non-ferrous metal processing such as aluminum alloys below the micron size mainly uses single crystal diamond tools, and single crystal diamond tools can be used for cutting probes or probes with nanometer precision. In order to improve the performance of cemented carbides, tool manufacturers are currently researching to make carbide grains more fine, and have achieved gratifying results. Ultrafine grained carbides with a particle size of 90 nm have been developed and the particle size has been experimentally produced. It is a 60 nm advanced superfine grained carbide.
Table 1 Properties of ultrafine grained cemented carbides WC particle size (nm)-hardness (HV)-elastic modulus (GPa)
300-1902-570
90-2361-600
In addition to tool materials, the geometry of the tool is critical to achieving micromachining. Under micro-cutting conditions, precise cutting of very thin materials requires an extremely sharp cutting edge, which is a very small cutting edge radius. Not only that, the sharpness of the cutting edge is also related to the quality of the cutting surface, the microstructure and the lattice dislocation. Accurate measurement of the tool edge contour is a prerequisite for the tool edge grinding and quality analysis of the fine cutting process. Micro drill or an end mill micro high hardness material, processing difficulties, low processing methods commonly used grinding wheel grinding efficiency, using FIB (Focused Ion Beam, focused ion beam), WEDGE (Wire Electro Discharge Grinding , wire electrode discharge grinding The method of making carbide micro drills or micro- milling cutters is very convenient and easy to meet the accuracy requirements. End mills with two-tooth, trapezoidal, semi-circular, inline, square and other shapes can be used for milling . Carbide shank milling cutters suitable for micro-machining have been widely used in the industry. High-precision fabrication of micro- milling cutters and drill bits requires high technical requirements. The smaller the diameter, the more difficult the production, the milling cutter with a minimum diameter of 0.1 mm and The drill bit has been able to produce. Among the hard alloy micro drills currently available on the market , the drilled drill has a minimum diameter of 0.03 mm and a flat drill of 0.01 mm. According to reports, in the laboratory using electrolytic grinding, a very small diameter drill bit of 0.005mm can be produced .
The micro-tools currently available on the market are extremely uneven in size and shape. For example, 31 drills with a diameter of 0.02 mm supplied by the same supplier were tested. The test results showed that the average diameter was 0.021 mm and the standard deviation was 0.0015 mm. The average thickness of the core was 0.0063 mm and the standard deviation was 0.0017 mm. This accuracy is obviously poor. Therefore, improving the manufacturing precision of micro-tools is one of the problems that micro-cutting needs to solve.
5 micro-cutting mechanism
The study of micro-cutting mechanism is of great significance for rational selection of cutting parameters, guaranteeing the quality of micro-cutting, reducing production costs and increasing productivity. In micro-cutting, due to the small size of the workpiece, it is not allowed to use a large depth of cut and feed in terms of strength and rigidity. At the same time, in order to ensure the dimensional accuracy of the workpiece, the thickness of the surface finish layer of the final finishing must be less than its accuracy. Value, so the amount of cutting must be small, such as the depth of cut is sometimes smaller than the grain diameter of the material, so that the cutting can only be carried out in the grain, then the cutting is equivalent to cutting one by one, the physical essence of cutting is Cutting the molecules and atoms between materials to remove atoms or molecules, the traditional cutting theory based on continuum mechanics is not suitable for micro-cutting. Therefore, the study of micro-cutting mechanism needs to adopt a different method from traditional plastic theory. research. The strain gradient plasticity theory is the extension and improvement of the traditional plasticity theory, and it is the necessary bridge between the classical plastic mechanics theory and the atomic simulation. In recent years, a variety of strain gradient plasticity theories have been developed. The typical CS (couple stress) strain gradient plasticity theory, SG (stretch and rotation gradients) strain gradient plasticity theory and MSG (mechanism-based strain gradient) strain gradient plasticity theory .
Using strain gradient theory, scale effect and dislocation effect can be predicted, and the results are consistent with the test. Microindentation, crack tip field, interface crack, filament twist and meager have been successfully analyzed in the field of micromachines and micro-components. Problems such as beam bending have begun to be applied in micro-molding research. Using strain gradient plasticity theory to study micro-cutting deformation will be the direction of micro-cutting mechanism research. In addition, the spindle rotation speed during micro-cutting is generally very high, and the machining precision is very precise. Therefore, micro-cutting has the characteristics of high-speed precision cutting. Applying the research results of high-speed precision cutting mechanism to the micro-machining field is also a trend of micro-machining research.
(1) Simulation of micro-cutting mechanism
Mainly using finite element technology and molecular dynamics method, finite element technology is based on continuum mechanics, so molecular dynamics method is more suitable for micro-cutting. Molecular dynamics simulation of micro-cutting mechanism has been carried out for more than ten years in the world. The research work is mainly to establish cutting models at the atomic and molecular scales to understand the chip and surface formation process from an atomic and molecular perspective. Explain the effects of material properties, tool geometry and process parameters on micro-cutting stress and strain distribution, cutting forces, cutting temperatures and machined surface quality.
(2) Minimum cutting thickness
The minimum effective cutting thickness that can be used for stable cutting is called the minimum cutting thickness. Chip shape, cutting force, cutting stability, micro-machining of workpiece materials, reasonable selection of cutting amount, and surface quality of machining are all affected by minimum cutting thickness. Therefore, the study of minimum cutting thickness is of great significance for micro-cutting. . The minimum cutting thickness that can be achieved by micro-cutting is related to the arc radius of the cutting edge of the tool, the physical and mechanical properties of the workpiece material, the microstructure and the friction coefficient between the tool and the workpiece in the third deformation zone. Since the minimum cutting thickness has many influencing factors, it is difficult to determine the minimum cutting thickness. In the actual production, the minimum cutting thickness is generally determined according to the radius of the cutting edge of the cutting edge. The research shows that the minimum cutting thickness is proportional to the radius of the cutting edge of the tool. The proportional coefficient is related to the tool and the workpiece material, generally 0.165~0.246. If the cutting edge radius is 50nm, it is necessary to achieve ultra-thin cutting thickness. Micro-cutting, the minimum cutting thickness at this time is about 10 nm.
(3) Chip form
Chips can only be produced when the depth of cut of the micro-cut is greater than the minimum cut thickness. Similar to conventional cutting, micro-cutting chips have three forms: continuous chips, non-continuous chips, and chips associated with built-up edges. The shape of the chip is related to the performance of the workpiece material, the cutting speed, the cutting deformation, and the like.
(4) Micro-cutting force
The cutting force during micro-cutting is small, but the unit cutting force is large, and the depth-of-depth resistance is greater than the main cutting force. The cutting force increases as the depth of cut decreases, and the cutting force increases sharply when the depth of cut is small, which is the size effect of the cutting force. The existence of the cutting force size effect makes the cutting force model of ordinary cutting not suitable for micro-cutting. The size effect of the cutting force is closely related to the cutting edge radius. Due to the radius of the cutting edge, the cutting edge forms a large negative rake angle during micro-cutting, which increases the cutting deformation and increases the unit cutting force during cutting. Big. If the depth of cut is further reduced, the cutting may occur inside the crystal grains. At this time, the cutting force must be larger than the molecular and atomic bonding force inside the crystal, so that the cutting force per unit cutting area is sharply increased. The cutting force during micro-cutting is also related to the crystal orientation and grain boundaries.
(5) Cutting temperature
Due to the small amount of cutting used for micro-cutting, the cutting temperature of micro-cutting is lower than that of conventional cutting. For micromachining with high precision requirements, the influence of the change of machining temperature on the machining accuracy can not be ignored, and the influence of cutting temperature on the wear of micro-cutting tools can not be ignored.
(6) Micro-processability of workpiece materials
The removal process of the workpiece material depends not only on the cutting tool, but also on the workpiece material itself. The micro-machining properties of micro-cut workpiece materials can be defined by nano-scale surface roughness and negligible tool wear over a certain machining distance. Factors affecting the micro-machineability of the workpiece material include the affinity (chemical reaction) of the workpiece material and the tool material, the crystal structure of the workpiece material itself, dislocations, defect distribution, and heat treatment state (such as the anisotropy of the polycrystalline material). Part processing surface integrity has a greater impact).
(7) Tool deformation
The rigidity of the tool has a considerable influence on the micro-machining process. For example, when the rigidity of the tool is insufficient in the milling process, the machining accuracy will be deteriorated during the machining process, and the micro- cutter will be broken when it is severe . The tool of the micro end mill is deformed to
δ=F·L3/3·E·I
Where δ is the radial deformation of the end mill; F. Radial cutting force; L is the length of the tool extends; E is the elastic modulus of the tool material; I (I = πD4 / 64 , D is the mill The equivalent diameter is the pole moment of inertia of the tool.
(8) Surface roughness and cutting stability
The surface topography of the workpiece is the result of the contour of the tool being mapped onto the workpiece. Therefore, the surface roughness of the machining is determined by the accuracy of the relative motion between the tool and the workpiece and the shape of the cutting edge of the tool. In the micro-cutting, if the cutting depth is smaller than the grain diameter of the workpiece material, it is equivalent to cutting one discontinuous body, the microscopic defects of the workpiece material and the unevenness of the material distribution, etc., so that the cutting force of the tool during micro-cutting is changed. Large, the cutting edge is subjected to large impact and vibration. The effect of vibration in micro-cutting on the quality of the machined surface cannot be ignored.
(9) burr
The burr is a tiny protrusion formed on the surface of the workpiece due to plastic deformation after cutting. The presence of burrs affects the fit of the part and reduces the dimensional accuracy and surface quality of the workpiece. The use of burred parts poses a safety hazard, especially in certain special applications such as aerospace. Therefore, it is necessary to increase the deburring process, and the methods of deburring include mechanical methods, thermal energy methods, chemical methods, electrolysis methods, electrochemical methods, grinding methods, and the like.
(10) built-up edge
The influence of built-up edge on micro-cutting can not be ignored. The built-in edge of cold welding on the cutting edge will cause the geometric angle of the tool to change, affecting the cutting force and cutting deformation. The built-up edge will also affect the surface roughness of the machined surface. The production of built-up edge is affected by the microscopic defects of the blade, the cutting speed and the feed rate. In micro-cutting, the lower the cutting speed, the higher the built-up edge, and the smaller the feed amount, the higher the built-up edge.
(11) Tool wear
Similar to conventional cutting, there are two forms of failure of micro-cutting tools: wear and chipping damage. The deformation of the three deformation zones, in particular the tool-to-workpiece friction in the third deformation zone and the mechanical recovery of the tool due to the elastic recovery of the machined surface. At the beginning of the cutting, the tool has initial micro-wear, and after a certain period of cutting, the tool wear will gradually increase, and sometimes it will suddenly deteriorate. Tool wear occurs mainly on the front and back knives of the tool. Due to oxidation, diffusion, etc., the tool also produces thermochemical wear. The chipping damage occurs when the stress on the cutting edge of the tool exceeds the local bearing capacity of the tool material. It is the most difficult to predict and control damage, and the effect on the quality of the machined surface is greater than the influence of the front and back face wear. Lowering the cutting temperature reduces tool wear.
6 micro cutting CAD / CAM technology
Cimatron E is a commercial CAD/ CAM software for micro-cutting, primarily for micro-milling. Since April 2003, the European Financial Community has begun to fund CRAFT, which has been micro-milling research on injection molds for micro-plastic components for 24 months. The project involved the entire process of micromachining technology, including Fraunhofer Institute of Production Technology (IPT), CAD/ CAM software supplier Cimatron Gmbh, milling machine manufacturer Kern, tool manufacturer Magafor, and mold maker Promolding BV Structoform And MMT AG). The hardness of the mold material is 53HRC, the precision of micro-die milling is <5μm, and the surface roughness Ra<0.2μm. Tool manufacturers offer tool diameters of up to 50μm, and milling machine toolmakers offer spindle spindles with spindle speeds up to 160,000 rpm. CAD/ CAM software vendors offer Cimatron E software for micromachining.
Unlike pure solid modeling, Cimatron E's solid-surface hybrid modeling technology uses the CAD function of “design for manufacturing†to repair geometric models, fuse gaps and become solid through various surface functions, and its ACIS core technology provides up to 1 nm. The internal precision meets the special requirements of micro milling. In order to reduce the risk and prevent discontinuous micro-curved surfaces generated during tool change, Cimatron E offers a variety of micro-milling strategies. Slash or spiral lower knives are supported in the NC strategy to ensure maximum smooth and continuous entry of the tool into the workpiece. A uniform tool path is obtained during the machining process by applying a high-speed cutting (HSC) strategy, and the knowledge of blank residue is used to prevent the knife from being broken to open the micro-cavity. Cimatron E's micro-milling technology ensures efficient and safe tool path by identifying real residual micro-blanks and the same functions of roughing, secondary roughing, finishing micro and cycloidal roughing. High-hardness materials and high-quality curved surfaces require 5-axis simultaneous cutting for very small diameter short-cone tools.
In order to meet the requirements of high-speed micro-milling, Cimatron E uses a variety of high-speed milling strategies, such as corner fillet joints, zero overlap cycloidal finishing, S-joint and spiral lower cutters, adaptive Z-layer finishing and streamlines machining. Cimatron E also supports spline approximation machining and streamline milling to reduce machining time and reduce tool wear and tear.
7 Development prospects of micromachining technology
Micro-mechanical is an important development direction, and its application prospects are very good. At home and abroad, it is very concerned about research in this field. Micro-machining technology is one of the most active research directions in the field of micro-machine manufacturing.
At present, the research on micro-machining process and equipment is still in the exploratory stage as a whole, and has not yet formed a complete and mature technical system and technical capabilities for scale manufacturing. It is expected that in the next 15 years or so, small manufacturing processes and related equipment technologies will be rapidly developed, especially in micro-small weapons, micro-medical devices, biomimetic devices, detection devices, and aerospace devices. In the future, we should pay attention to the following topics in micro-cutting to promote the production and application of micro-cutting technology.
(1) Basic research on micro-cutting applications including research on key technologies of micro-part cutting equipment, mainly researching high-speed spindle systems, positioning, motion and control technology of precision worktables, composite micro-machining equipment and technology; micro-cutting tool materials and tools Research in production technology; rapid cutting of micro-cutting tools, workpieces, testing and monitoring techniques for micro-machining processes.
(2) The study of micro-cutting mechanism mainly studies the micro-cutting uneven deformation field under the thermo-mechanical coupling stress, studies the constitutive equation of the workpiece material at the micro-scale, and analyzes the size effect, uneven strain and position of the micro-cutting deformation zone. Influence of error on shear deformation stress and shear deformation energy; study of the influence of minimum cutting thickness on chip shape, formed surface formation, cutting force, cutting temperature, etc. and surface roughness and subsurface damage of workpiece material microstructure The impact of the establishment of micro-machining theory and technical system; research multi-scale micro-machining simulation technology, laying the foundation for the application of micro-machining technology.
(3) Micro-cutting process research includes micro-machining process of various new materials such as steel, titanium alloy, stainless steel, aluminum alloy, ceramics and other non-metal materials and various composite materials, and micro-cutting CAD/ CAM technology.
(4) Research on the economics and reliability of micro-machining technology.
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